The diagnostic reliability of US was investigated in the study of cryptorchidism during a 9-month clinical and US trial performed on 52 children (mean age 4.3 years). Cryptorchidism was unilateral in 44 cases (24 on the left side and 20 on the right side), and bilateral in 8 cases. Of 60 cryptorchid testes, 53--belonging to 45 of 52 patients--were located in the inguinal region (18 along the canal, 20 at the external ring, and 15 at the internal one); the others 7--belonging to the remaining 7 subjects--were found in an extra-inguinal region (4 in paravesical location and 3 in the inferior abdomen). US confirmed 29 undescended testes with clinical evidence in the inguinal region and identified, in the same area, 24 other testes which had been missed at clinical examination. In addition, US provided useful information as to both structure and volumetry of the gonads. On the other hand, US failed to provide reliable diagnostic results in the rare cases of pelvic and abdominal cryptorchidism, where precise testes localization was achieved by CT. US is the diagnostic technique of choice in the study of cryptorchidism (after clinical examination): the method is non-invasive and simple, has low cost and its use is widespread; moreover, US does not administer ionizing radiations. According to our experience, US had 88.3% diagnostic sensitivity as for undescended testes (53 of 60), and 100% specificity. In addition, US monitoring allowed clinical efficacy of therapeutic trials to be verified in 25 patients (17 sensitive to therapy): 16 of them were treated with human Chorionic Gonadotropin (hCG) and 9 with Luteinizing Hormone-Releasing Hormone (LH-RH).